Manipulation of nitrogen-contaminated natural gases



Dec. 7, 1954 l.. s. TwoMEY 2,696,088

MANIPULATION oF NITROGEN-CONTANINATED NATURAL GAsEs Filed Aug. 4, 1949 4sheets-sheet 1 INVENTOR Dec. 7', 1954 l.. s. TwoMEY n 2,696,088

MANIPULATION OF NITROGEN-CONTAMINATED NATURAL GASES Filed Aug. 4, 1949 4Sheets-Sheet 2 L. S. TWOMEY Dec. 7, 1954 MANIPULATION OF'NITROGEN-CONTAMINATED NATURAL GASES Filed Aug. 4, 1949 4 Sheets-Sheet 5INVENTOR Dec. 7, 1954 s. TWOMEY MANIPULATION OF NITROGEN-CONTAMINATEDNATURAL GASES 4 sheets-Sheet 4 Filed Aug. 4, 1949 INVENTOR United StatesPatent MANIPULATION 0F NIT ROGEN-CNTAMINATED NATURAL GASES Lee S.Twomey, Vista, Calif.

Application August 4, 1949, Serial N0. 108,631

8 Claims. (Cl. 62-175.5)

This application relates to the transportation, purification, storageand distribution of natural hydrocarbon 'gases initially contaminated bymaterial proportions of nitrogen.

Certain portions of the United States, notably western Kansas,southwestern Colorado, and the Texas Panhandle, produce great quantitiesof natural gas containing up to forty per cent by volume of nitrogen.The great part of this gas finds a market only at a considerabledistance from the field and must be transported through pipe lines forhundreds of miles, at a cost which often materially exceeds the value ofthe gas at the well-head.

The separation and rejection of part or all of the original nitrogencontent has important advantages, even when this step is performed atthedelivery end ofthe transmission line, andr even greater' advantageswhen the vremoval is effected before the gas is transported over a greatdistance. The step is particularly effective and advantageous whencombined with storage of part of the purified gas ata point more or lessadjacent to that at which the gas is distributed and used, or when the'step of purification is combined with the recovery of liquidhydrocarbons' from the purified gas. The nature of these advantages andthe various manners in which they may best be realized will be referredto in detail hereinafter.

Various lmethods for separating theY contaminating nitrogen from thenatural gas are available, lthe present specification 'describing onlythe general Amethod in which separation is effected by liquefaction ofthe entire feed stream and fractionation of the resultantliquid in asuitable column. This method of separation may be employed either in thefield or at the delivery end of a long distance transmission line, or atK4some convenient intermediate point, and may be combined with storageof part or all of the purified gas and with the recovery of valuableliquid hydrocarbons from the gas.

VThe invention may best be described with reference to the attacheddrawings and the'following description thereof, in which:

Fig. l is a diagram illustrating the essential steps of the'process,devoid of detail and describingvarious permissible alternatives ofprocedure;

Fig. 2- is a fiow-sheet of an operationand assemblage of apparatus forperforming the actualseparation of nitrogen and providing the extraneousrefrigeration required by ther system, the purified gas being stored inliquid form;

Fig. 3 illustrates a modification of the operation in ywhich thepurified gas is `obtained in the form of a vapor at low pressure and isrecompressed `for delivery into a long distance transmission line, and

Fig. 4 illustrates another modification in which the purified gas,delivered from the column as a liquid, is pumped in liquid form througha vaporizing interchanger and thus 'delivered into the transmission lineunder the pressure created by the liquid pump.

Referring first to Fig. l, A indicates a gas field producingnitrogen-contaminated natural gas; B is a dehydrating unit in which thegas is deprived of carbon dioxide, hydrogen sulfide and water vapor; Cis a fractionating system as described in detail in connection withFigs. 2, 3 and 4; 'D is a gas storage system and E is a distributingsystem such as a city gas service. The locations of elements A and Eare, of course, i'ixed by circumstances and not controllable.

The other three principal elements may be located as convenient: thus,the treatingfunit must be ,between p in degrees Kelvin.

the field and the fractionating plant but may be adjacent to either ifthey are separated; the fractionating plant and the storage plant (ifprovided) may each be adjacent to the field or adjacent to thedistribution area or at a medial point, and finally, the fractionatingplant and the storage plant may be closely adjacent or may be separatedby any convenient distance.

Fig. l shows a line F-F connecting the field with the fractionatingplant, with treating unit B located anywhere between A and C; a gas lineG-l-I connecting the fractionating plant with the storage plant; aliquid line O connecting the fractionating'plant with the storage plant,useful only if these two elements are closely enough adjacent to permitthe transfer of a liquefied gas in liquid forni; a gas line G-K-lconnecting the fractionating plant directly with distribution andbypassing storage; a gas line I-l connecting storage with distribution;a gas line lil-A1 connecting the field with the distribution area andbypassing storage. With such lines, of lengths determined by therelative locations of the units, it is possible to take care of anydesired alternatives of procedure.

The line indicated at Z is for the purpose of introducing a diluent gasas later described. Raw gas from the field may be used as a diluent byintroduction to line l through line A2.

The fractionatir'ig system illustrated in detail in later figuresconsists first of a series or group of interchangers L in which the feedgas is refrigerated, the cooled or liquefied gas passing to aconventional fractionating column M, for example, one provided withbubble plates, in which a desired portion of the nitrogen of the raw gasis removed as a top cut, which effects part of the cooling of the feedgas in L and is vented at N. The column bottoms, a liquid richer inhydrocarbons than the feed gas, may passthrough a relatively shortconnection O, in the liquid condition, to a liquid phase storage vesselP from which the liquid is withdrawn as required through a vaporizer Qto be sent to distribution. Or the bottom liquid may pass through aconduit R to refrigerating unit L in which it is vaporized in effectingpart of the cooling of the feed gas, passing thence in gaseous fornithrough G--H to low pressure gaseous storage S or high pressure gaseousstorage T, or through G-K-l to the distributing system E. Or as a thirdalternative, the column bottoms may be directed to a refractionatingcolumn U from which a bottom cut consisting of propane and heavierhydrocarbons is withdrawn at V while the top cut, consisting mainly ofmethane and ethane, passes to conduit G and thus to gaseous storage in Sor T, or to a reliqueier W which places it in liquid storage P, ordirect to distribution system E. In instances in which thefractionation'plant and the storage system are separated by a distancetoo great for the transmission as a liquid of the bottom cut from columnM, reliqueier W will take care not only of the top cut from column U butalso of the revaporized bottom cut from column M.

Any make-up refrigeration required in L will be supplied by theevaporation of a liquefied gas, such as liquid methane, introduced froman extraneous source at X and returned from Y to its source.

Referring now to Fig. 2, illustrating a method which delivers thepurified gas in liquid form into liquid storage: an actual pipe linesupply of nitrogen-contaminated fuel gas is taken by way of example,this gas containing 90.7% of hydrocarbons, mainly methane, and 9.3% oflower boiling components, almost entirely nitrogen. In the ensuingdescription, percentages are in mol per cents, pressures are inatmospheres absolute and temperatures All the figures given are closeapproximations, fractions being substituted by the nearest round figure.It will be understood that the pressure and temperature relationsrecited are illustrative and not limiting. They will vary to some extentwith changes in composition of the gas and may be varied,

, within limits, at the will of the operator.

Y not necessarily be the delivery end of a long distance transmissionline. This gas enters an interchanger 14 in which it is cooled to about285 by separated nitrogen leaving the system. It then passes throughconduit 15 to interchanger 16 in which its temperature is reduced toabout 134 by an expanded and evaporating stream of liquid methaneproduced by a cascade liquefying system later described. At thistemperature and at substantially the original pressure of l2 atmospheresthe gas is about 93% liquefied.

The partially liqueiied stream passes through conduit 17 to a boilingand condensing coil 18 immersed in a pool 19 of liquid, substantiallypure methane in the base of a fractionating column 20 (coiumn D of Fig.l). In this coil liquefaction is completed, the liquid stream passingthrough conduit 21 and expansion valve 22 and entering the column at amedial height as at 23.

It should be understood that While liquefaction of the feed stream priorto entry into the column is desirable, as restricting the column to aminimum size, it is entirely possible to feed to the column a partiallylique ied feed stream, or even a gaseous stream. in such cases theliquefaction requisite for fractionation being produced Within thecolumn by increasing the quantity of reflux liquid.

The column may be of the single stage type and may be maintained at 3atmospheres pressure. With a suiiicient number of effective plates, thetemperature in pool 19 will be about 125 and the vapor temperature atthe upper end of the column about 89 K. The composition of the vaporvented at 24 Will be about 99% nitrogen and 1% methane.

The vent vapor is divided at the column outlet, a portion passingthrough conduit 25 to a nitrogen liquefying cycle later described Whilea quantity equal to that momentarily separated from the gas feed passesthrough conduit 26 to an expansion valve 27 by which its pressure isreduced to about l atm. and its temperature to about 86. The vent gasthen passes through interchanger 14, in which its temperature is raisedto 290 in effecting the iirst cooling of the gas feed, and is dischargedfrom the system at 2S.

The column is provided with retiux liquid by a nitrogen liquefactioncycle taking gas from the top of the column through conduit 25. The gaspasses first through an interchanger 29 in which its temperature israised to 305 in cooling a compressed and Water-cooled nitrogen stream,then through conduit 30 to two stages of compression 31 and 33 withinterposed water-cooling at 32 and inal Water-cooling at 34. TheWater-cooled stream, at a pressure of 25 atmospheres, is cooled to 129in interchanger 29 in heating the stream of nitrogen passing to the irststage of compression.

The refrigerated nitrogen then passes through conduit 36 to interchanger37 in which it is cooled to 120 and is liquefied by interchange againstcold methane vapor from a source later described. The liquefied streampasses through conduit 38 and expansion valve 39, by which it is reducedto column pressure, and enters the upper end of the column in which itfunctions as redux liquid.

Returning now to the bottom of the column the liquid methane collectingin pool 19 passes through conduit 40 to interchanger 41 in which it iscooled by interchange against expanded and evaporating liquid nitrogendrawn from the nitrogen liquefaction cycle previously described.. Thecooled liquid then passes through conduit 42 and expansion valve 43 toan insulated storage tank 44 which may be maintained at 1.15atmospheres, at which pressure the temperature of the liquid will beabout 113 K. The composition of the liquid entering the tank isapproximately 0.2% nitrogen and 99.8% methane and heavier hydrocarbons.ln this tank the liquid is maintained in storage until required, atwhich time, it is withdrawn through conduit 45 to be vaporized anddistributed.

If preferred, the liquid may be withdrawn from storage by a pump 146adapted to handling liquids, by which it is raised to some requiredtransmission line pressure, then vaporized as at 1.47 and introducedinto a, transmission line 148 leading to a distribution system.

Due to the reduction in pressure at expansion valve 43 there is a smallamount of flash from the liquid as it enters the vessel, usually about6% of its weight. This vapor passes through conduit 46 and interchanger47, in which its temperature is raised to 305, then through conduit 48to a compressor 49 which raises the pressure to 3 atmospheres, through awater-cooling step 50, through interchanger 47 in which it is cooled toabout 132, and iinally through conduit 51 to the column feed at 23.

The production of flash vapor in the storage tank may be avoided bysuiiiciently extending the aftercooling of the column liquid by expandedand evaporating liquid nitrogen, in which case elements 46, 47, 48, 49,50 and 51 will not be required, This liquid may be drawn Jfrom conduit38 through branch conduit 156 and expanded by valve 151 intointerchanger 41, the vaporized nitrogen returning through conduit 97 toa junction with conduit 25 leading to interchanger 29.

The refrigeration required in the above steps is provided in part by theexpansion of the gas feed from intake pressure to column pressure, inpart by the nitrogen cycle above described, and in part by a cascadesystem including an ammonia cycle, an ethylene cycle and a methanecycle.

Starting at the right hand end of Fig. 2, the ammonia cycle comprises atwo-stage compression unit 52 and 54 with intercooling at 53 andaftercooling at 55, the pressure being raised to about 4 atmospheres inthe rst stage and to about 15 atmospheres in the second. At the latterpressure the ammonia is liquefied in the aftercooler at 311 and passesthrough conduit 56 into a receiver 57. The liquid then passes throughconduit 58 and expansion valve 59 into a ilash tank 68 maintained atabout 4 atmospheres and 272 K. The flash from this tank returns throughconduit 61 to the intake of second stage compressor 54.

The flashed liquid ammonia passes through conduit 62 and expansion valve63, by which its pressure is reduced to 1.15 atmospheres and itstemperature to 245, to an interchanger 64 in which it liqueiies ethylenein the next stage of the cascade. The ammonia vapor returns at about 260through conduit 65 to the intake of rst stage compressor S2.

The ethylene cycle includes a twostage compression unit 66 and 68 withintercooling at 67 and aftercooling at 69, the pressure being raised toabout 5 atmospheres in the first stage and to 22 atmospheres in thesecond. The compressed gas leaves the aftercooler at 311 and passesthrough conduit 70 to interchanger 64 in which it is liquefied at 248 byexpanded and evaporating liquid ammonia. The liquefied ethylene passesthrough conduit 71 into a receiver 72 and thence through conduit 73 andexpansion valve 74 into a flash tank 75 maintained at about 5atmospheres, and 201. The flash from this tank returns through conduit76 to the intake of second stage compressor 68.

The liquefied ethylene passes through conduit 77 and expansion valve 78,by which its pressure is reduced to 1.15 atmospheresV and itstemperature to 171, to interchanger 79 in which it liquees methane inthe third stage of the cascade, the ethylene vapor returning at about260 through conduit 80 to the intake of iirst stage compressor 66.

The methane cycle includes a two-stage compression unit 81 and 83 withintercooling at 82 and aftercooling at 84, the pressure being raised to6 atmospheres in the first stage and to 28 atmospheres in the second.The compressed gas leaves the aftercooler at 311 and passes throughconduit 85 to interchanger 86 in which it is cooled to 290 byinterchange with a returning stream of once-used methane. The partiallycooled gas passes through conduit 87 to interchanger 79, in which it isliquefied at 176 by an expanded and evaporating stream of liquidethylene.

The liquefied methane passes through conduit 88 to a receiver 89 andthence through conduit 90 and expansion valve 91 to a ash tank 92maintained at 6 atmospheres and 139. The tiash from this tank returnsthrough conduit 93 to the intake of second stage compressor 83.

The liquid methane thus produced supplies refrigeration to the raw gasliquefying and fractionating system at two points.

A stream of the liquid passing from flash tank 92 through conduit 94 isdivided, the smaller portion passing through conduit 95 and expansionvalve 96, by which its pressure is reduced to 1.5 atmospheres and itstemperature to 118, to interchanger 37 in which it effects the describedliquefaction of nitrogen, passing i thence through `conduit '98 tointerchanger V86, inwhich lit'effectsthe firstcooling of" cascademethane vapor, and vreturning through 'conduit 91H0 the intake .ofmethane compressor 81 at 270.

The remaining quantity of liquid methane passes "through conduits 94vand 100 and expansion valve 101, v'by'which its pressure `is vreducedto 1.4v atmospheres and its temperature to 117 K., to intel-changer 16in which it 'produces the described partial liquefaction of .thedeihydrated gasr feed. The vapor 'from this expansion andinterchangereturns through conduit 102 yto the first vstage methanecompressor 81.

Fig. 3, to which reference is now made, illustrates -a modification ofthe method of Fig. 2 in which the purifiedgasiis delivered into atransmission line or distribut- 'ing system at column pressure or at ahigher pressure produced by recompression of the product gas.

The'dehydrated gas supplyenters the system through :conduit 10, yand isdivided :into two streams passing in parallel through interchangers 110and 143. These :streamsare rcooled to about 134 K. and partiallyliquefied by an expanded and evaporatingstream ofthe liq-`uid,princip'a1ly methane, Withdrawn `from the bottom of -fractionatingcolum and by' vent nitrogen from the top of the column. The partiallyVliquefied streams are merged'in conduit 111 andipass to vboiling andcondensingcoil .19 in which liquefa'ction is completed except forpossible difiicultly liquefiable gases such as neon or helium, thelatter a fairly'common component of the nitrogen-containing naturalgases. Ifthese are present the stream may be `passed 'through conduit112 to a separator 113 from which uncondensed gases are vented -at 114.The liquid then passes through conduit 21 and expansion valve 22, bywhich kit 'is reduced to column pressure, to the medial point 23 atwhich it is introduced intothe column. As before described, this may bea'single stage column, provided Awith bubble plates, or other form ofVfractionating 'column as may be preferred, `and is desirably maintainedat about 3 atmospheres absolute.

The liquid collecting in the bottom of the column, consisting ofthehydrocarbons originallypresent in the `gas, together with a 'minuteYresidue of nitrogen, passes in greater part through conduit 115 andexpansion valve V116,y by which itA is reduced to slightly overatmospheric pressure, to interchanger 110, in which it is heated toapproximately thek temperature' at whichl the feed gas 7enters thesystem. The warmed gas Vpasses kthrough `conduit 117 to a` gascompressingunit generally indicated at 118', in which the lpressure israised to that required `to introduce the gas into a long 'distancetransmission line -1-19. In case the fractionating plant is located 'at.the delivery end of the line a single stage compression unit at 118mayv suffice vfonintroducing the gas directly -into a distributionsystemfor into gaseous 'storage for later distribution.

The remainder of the 'liquid stream from the column is diverted throughbranchconduit v120 land expansion -valve 121 into interchanger '122 inwhich it is yvaporized and brought up to substantially.atmospherictemperavture in liquefying a stream of compressed nitrogen. The warmedgas from this interchange vpasses through. conduit 123 to a point ofVjunction kwith conduit 117 vand thus to compressor 118.

Liquid nitrogen for refiuxing the column is provided by a nitrogenliquefaction cycle-differing somewhat from that described in connectionwith Fig. 2. Thestream of cold nitrogen leaving the top of the column at24 is divided, `a quantity suiiicient to provide the reflux required bythe column passing through conduit 25 to interchanger .29, in which itis brought up to atmospheric temperature, and thencethrough conduit'30to the two-stage compression and cooling=unitr3-1-p3233 l34 in which thepressure is raised to 25 atmospheres. The compressed stream iiowingthrough conduit 35 isdivided, one portion passingthrough valve 124 intointerchanger 122in which it is cooled andv liquefied by an expandedstream of column bottom product. The yremaining portion passes'throughvalve 125 into interchanger 29 in which it is cooled by the nitrogenstream passing toward the compressor, the cooled stream passingthrough`conduit 36 to Vinterchanger .37 in which it isl liquefied by anexpanded .and 'evaporating lstream of 'liquid fmethane. The 'two'streamsof #liquid :nitrogen 'gen vent 144.

pass Vthrough :conduits 126 and 38 to expansionV :valve y'39 and thus'into the' upper end of the column.

Dependent on the composition of the feed gas and the closeness offractionation, three alternatives are `available in the handling of thenitrogen stream entervcooled and joins, in conduit 111, the portion ofthe gas feed'cooled in exchanger 110. v

In this alternative, the vent nitrogen `has no part in liquefying thecascade ethylene, which liquefied by exchange withboilingliquidarnnioniain exchanger 64,'the

parallel interchanger 129 (used in the second alternative) vbeingitheninoperative.

As it is'desirable'to control rather closely the enthalpy `ofv themerged stream entering'coil 18 from conduit .111,

there are sonie'conditions of feed composition and closenessof'fractionation under which it is uneconomical 'or impossible to lpassallor even any part of thevent nitrogen from conduit127 vthroughexchanger 143. This leads tothe second alternative in which a portion ofthe vent nitrogen takes the ypath just described, while the remainderpasses through expansion valve .130 into inter- `changer 129, where itis heated in liquefying either a portion or all of the cascade ethylene,thence passing @through conduit 131 vtonitrogen 132. In the Vevent thatYthe quantity yof nitrogen available for passage through 129 isinsufficient to liquefy all of the cascade ethylene, theexcess ofethylene is vliquefied by exchange with boiling liquid ammonia inexchanger 64.

In the third alternative, the entire quantity of vent nitrogen is passedthrough exchanger 129 and is heated inliquefying ethylene, finallypassing out through. nitrogen vent `'132, exchanger 143 meanwhile beinginoperative, with allot-theY gas feed passing through interchanger`In`this alternative, ammonia may or' may not be yrequiredin^exchanger64ffor liquefying part of the ethylene, depending on theamount of ethylene which the vent nitrogen vis abletor liquefy kinexchanger '129.

The cascade system of Fig. 3 differs from that of Fig. 2 inboth-themethane and the ethylene liquefying stages. In Fig. 2 a singleethylene interchangerl is provided, the liquefaction of compressedethylene being produced .solely by expanded and evaporating liquidammonia. ln Fig. 3 the stream of compressed ethylene delivered bycompressor 68 is divided, a portion passing through valved conduit 71 tointerchanger 64, in which it is liquefied -by boiling ammonia. Theremainder of the com- 'pressed ethylene passes through a valved branchconduit 128 =into an interchanger 129 in which it is liquefied bygaseous nitrogen flowing from column 20 Ithrough conduit 127 `and'expansionvalveli). The nitrogen, warmed by-this interchange, passesout'of the system as described. .The ethylene liquefied by theseinterchanges flows through conduits 71 and 133 into receiver 72,thereafter `taking Athe -course previously described.

The methane cycle differs from vthat of Fig. 2 in two respects.Thus,intercha1igers 86 and 79 are arranged in parallel, 'both deliveringliquid-methane' into receiver 89, yintei'clianger -86 being suppliedwith gaseous methane through branch conduit 133 and draining throughconduit 134. Nitrogen :iiquefier 37 is cooled by liquid methane .passingto it from flash tank 92 through conduit 135 and eiqzuansion valve 136,the ymethane vapor resulting from the interchange returning tocompressor 81 through conduit-98, interchangereofand conduit`99; Thecooling of ther nitrogen liquefier is the only use-of liquid methane inthis modification of the invention and the capacity of the cascadesystem is correspondingly reduced.

Fig. 4 illustrates a modification of the invention in which a pumpadapted to raise liquefied gases to a high pressure replaces thecompression unit 118 ofv Fig. 3, the .column liquid being passed througha 'Vaporizing inerchanger on its way to the intake of a transmissionine.

.Referring .to Fig. 4, the liquefaction of the gas feed is effected ininterchangers 14 and 16'and condensing coil 1S by interchanges withvgaseous column nitrogen, expanded cascade methane and boiling-columnliquid, lthe iirst two interchanges being inparalleland thethirdinseries with theltwo. Thus, the feed stream passing throughconduit 1 0 1s divided between interchangers 14 and 16 inproportion tothe amount of refrigeration available in each, the

rst being cooled by vent nitrogen from the column, passing throughconduit 26 and expansion valve 27 and being vented, after warming byinterchange, through n1- trogen vent 129. The second interchanger 16 iscooled by cascade methane passing through conduits .94 and 100 andexpansion valve 101, the expanded and warmed methane returning tocompressor 81 through conduit 102. The refrigerated gas, which may bepartly liquid, leaves the interchangers through conduits 1S and 17, thelatter leading to coil 18 from which it passes through conduit 21 andexpansion valve 22 into the column. The feed entering the column may bewholly liquid, or partially liquefied, or even gaseous, liquefaction inthe column of any vaporous feed being produced by increasing the supplyof reflux liquid over that required for fractionating a liquid feed.

The liquid collecting in the column, consisting of normally gaseoushydrocarbons together with a reduced and ordinarily very smallproportion of nitrogen, is preferably cooled below column temperature ininterchanger 4t, passing thence to a pump 140 capable of raising it, inthe liquid form, to whatever pressure is required at the transmissionline intake. The liquid passes from the pump through conduit 46 tointerchanger 86, in which it is vaporized without substantial change inpressure in liquefying part of the required supply of cascade methane,the high-pressure puried gas passing thence directly into transmissionline 119.

The initial cooling of the reflux nitrogen is effected by gaseous columnnitrogen on its way to compressor 31, as previously described, and theliquefaction of the cooled nitrogen by expanded cascade methane drawnfrom conduit i) through branch conduit 137 and expansion valve 138 andreturned to compressor 81 through conduit 139 and 99.

The cascade system of Fig. 4 is identical with that of Fig. 2 in theammonia and ethylene cycles and with that of Fig. 3 in the methaneliquefying cycle.

Numerous and important advantages are realized from the removal of amaterial part of the nitrogen prior to transmission of the gas to adistant point:

(a) The therm transmitting capacity of any given pipe line is increasedby the removal of the inert diluent and the concentration of theoriginal fuel value of the gas into a smaller volume and weight;

(b) The horse power required to transmit the gas over a long distance isconsiderably reduced, both by reduction in quantity of gas which must betransmitted per unit of heat transmitted and by reason of the more readycompressibility of the hydrocarbon-enriched residue;

(c) An important saving in cooling water consumption is effected, byreason of the higher temperature of nitrogen at any given dischargepressure and the elimination of the nitrogen;

(d) The thermal storage capacity of the line itself and of anyadditional storage vessels which may be provided are increased inproportion to the quantity of nitrogen removed;

(e) The removal of the nitrogen, if present in material proportion inthe field gas, often or usually permits the separation and recovery as asalable product of the higher boiling hydrocarbons (propane and heavier)without reduction in the heating value of the gas or with themaintenance of a specified B. t. u. requirement;

(f) The removal of higher boiling hydrocarbons thus permitted oftengreatly improves line operating conditions, avoiding risk ofcondensation in and trapping of the line;

(g) The removal of nitrogen permits the use of gas from fields of whichthe product is initially of too low heating value to be usefulcommercially;

(h) The removal of nitrogen permits standardization of heating value ofa gas supply drawn simultaneously and in varying proportions from eldsor wells producing gases of different compositions;

(i) The removal of nitrogen and consequent increase in calcrific valuepermits attainment of higher ame temperatures which, in many industrialoperations greatly increases the efficiency of high-temperature heatingsteps, by increasing the range between the flame temperature and thetemperature to which the work must be brought;

(j) The removal of nitrogen makes it possible to increase the averagetherm load factor of the transmission line by permitting it to care fora larger average distribution load.

The step of removing nitrogen from contaminated natural gas isparticularly desirable in instances in which aiy part of the gas supplyis to be stored in liquid form, t us:

(k) The storage of inert material is avoided-a given vessel will holdmore therms by reason of the concentration of the original heating valueinto a smaller liquid volume;

(l) ln the more usual instance, in which the gas is deprived of nitrogenbefore long distance transmission and reliqueed for storage at thedelivery end of the line, the quantity of heat to be removed in thereliquefying plant is reduced;

(m) T he temperatures of reliquefaction and of storage are materiallyraised, avoiding the use of the extremely low temperatures which are themost costly to attain;

(n) Elevation of the temperature of the liquid-storage vessel reducesheat infiltration through any given vessel insulation and (o) Reducesembrittlement of ferrous materials used in storage vessels;

(p) Change in composition of the liquid, which follows from fractionalvaporization of nitrogen, is eliminated by nitrogen removal and (q)Materially less heat is required for the vaporizatilon of the liquidwhen required for use in the gaseous p ase;

(r) The minimum temperature encountered in revaporizing the liquid isincreased and the liability to freezing of the heating fluids used invaporizing is reduced.

The steps of removing a material proportion of nitrogen prior to longdistance transmission and of placing part of the transmitted gas instorage at the delivery end of the line, at times of less than averagedemand, to be drawn on to help meet demands greater than average, arehighly cooperative. Not only does the removal of the nitrogen increasethe transmitting capacity of the line and the storage capacity of boththe line and the deliveryend storage, thus permitting smaller pipe linesand storage units to carry a given load, but also the provision ofstorage capacity materially improves the functioning of thenitrogen-removal plant.

Demands of a distributing system for gas vary widely from day to day oreven from hour to hour, this variation being seldom less than three toone and often much greater. This variation in demand has, in the past,been compensated in various ways, as for example by packing the line(raising the line pressure and thus increasing the quantity of gas intransit), and cutting off socalled interruptible loads at times ofincreased domestic demand (involving major price concessions to suchindustrial users) and similar expedients.

Both the transmission line and the nitrogen removal plant function mosteconomically under an unvarying load. The provision of storage at thedelivery end of the line, in quantity sufficient to supply thedilference between average demand and maximum demand for the anticipatedperiod, permits the pipe line to deliver, and therefore to take from thenitrogen removal plant, a constant quantity. With this provision, boththe nitrogen removal plant and the pipe line need be only of suchcapacity and dimensions as to carry the average load, rather than themaximum, and both rst cost and operating cost are reduced.

Or, for a transmission line of fixed size, equipped with a storagefacility, the average therm load factor of the line can be increasedthrough its increased ability to meet peak demands, which enables it tosupply an increased average demand, because the average demand which theoperator may commit the line to supply is limited by the ability of thesystem to meet peak demands successfully.

In removing a relatively large proportion of nitrogen, even withsimultaneous separation of valuable liquids of high heating value, itmay occur that the caloriic value of the residual gas is raised abovethat required by local custom or ordinances. In such cases the overlyrich gas may be diluted back to the requirement by the admixture ofgases of lower or no heating value, for example, coke oven gas, producergas, nitrogen contaminated natural gas, combustion gases or air.

Given a source of fuel gas of relatively high thermal value and, at adistance therefrom, a uctuating demand 'i for a gas of relatively lowthermal value, economy in the investment and operating cost can beattained by transmitting the high thermal value gas to a point as closeas is convenient to the demand, storing, in times of reduced demand, aportion of the transmitted gas in its undiluted state, meanwhilediluting another portion of the transmitted gas with air, ilue gas, coalgas, coke oven gas, producer gas, water gas, or any suitable material inorder to reduce its thermal value to that required by the demand, andsupplying the demand with the diluted gas. In times of increased demand,high thermal value gas can be removed from storage, diluted as above,and directed to the demand to augment the supply available by dilutingthe current delivery of the line. In event of line deliveryinterruption, the gas removed from storage and diluted can entirelyreplace line delivery and dilution.

With a line of fixed size, the addition of such a storage facility andoperation according to the above method will permit the increase of theaverage therm load factor of the line, as compared with the transmissionof diluted gas With or without storage, or with the transmission of richgas, its dilution and storage in the diluted condition of a portion forsubsequent use.

Provisions are made for this operation in the showing of Fig. l by thecross-over line Z connecting the iield line F with conduit G-H leadingto the storage plant and conduit G-H--K leading to the distributionsystem, and by the injecting connection Z into conduit J ahead ofdistribution.

l claim as my invention:

1. A manipulation of natural hydrocarbon gas initially contaminated withnitrogen, comprising: refrigerating said gas; fractionating therefrigerated material by repeated contacts of downowing liquid withupliowing vapor; withdrawing from said fractionation a vapor enriched innitrogen and a liquid enriched in mixed hydrocarbons; liquefying aportion of said nitrogen enriched vapor and supplying it to thefractionation step as reux of higher nitrogen content than saidrefrigerated material, passing at least a major portion of the remainderof the vapor enriched in nitrogen at substantially the temperature ofwithdrawal, in heat interchange with the gas to be refrigerated andthereby producing at least a part of said refrigerating effect, andvaporizing said hydrocarbon-enriched liquid to produce a gas having asmaller nitrogen content than the original natural gas.

2. A manipulation of natural hydrocarbon gas initially contaminated withnitrogen, comprising: refrigerating said gas; fractionating therefrigerated material by repeated contacts between downowing liquid andupowing vapor and thereby separating a vapor enriched in nitrogen from aliquid enriched in hydrocarbons dividing said vapor while still at lowsubatmospheric temperature; liquefying a portion of said vapor andthereby providing reflux liquid for said fractionation; passing theremainder of said vapor in heat interchange with a stream of the naturalgas to be refrigerated and venting the resultant warmed vapor;collecting said hydrocarbon-enriched liquid in storage in liquid form,and subsequently vaporizing the stored liquid to produce a gas having asmaller nitrogen content than the original natural gas.

3. ln the refrigeration of a natural gas fractionating operation, thesteps comprising: dividing a stream of liquid methane into a firstportion and a second portion; vaporizing and superheating said firstportion in heat ex change with natural gas entering said operation,thereby imparting refrigeration to said operation and producing a rstmethane vapor stream; supplying a stream of methane vapor to becompressed compressing said last methane vapor stream to form a warmcompressed vapor stream; passing said second liquid portion in heatinterchange with a product of said operation and thereby forming asecond vapor stream at low subatmospheric temperature; passing said warmcompressed stream and said second vapor stream in heat interchangewherein said Warm compresed stream is cooled and said second vaporstream is superheated; combining said superheated vapor stream with saidfirst vapor stream to form said stream of methane vapor to becompressed, and liquefying the combined compressed streams to providethe liquid stream to be divided.

4. A manipulation of natural hydrocarbon gas initially contaminated withnitrogen, comprising: refrigerating said gas; fractionating therefrigerated material by repeated contacts between downflowing liquidand upflowing vapor and thereby separating a vapor enriched in nitrogenfrom a liquid enriched in hydrocarbons; dividing said vapor While stillat a low subatmospheric temperature; liquefying a portion of said vaporand thereby providing reflux liquid for said fractionation; passing theremainder of said vapor while at substantially the temperature ofWithdrawal in heat interchange with a stream of the natural gas to berefrigerated and venting the resultant warmed vapor and withdrawing saidhydrocarbon-enriched liquid.

5. A manipulation of natural hydrocarbon gas initially contaminated withnitrogen, comprising: refrigerating said gas; fractionating therefrigerated material by repeated contacts between downilowing liquidand upflowing vapor and thereby separating a vapor enriched in nitrogenfrom a liquid enriched in hydrocarbons; dividing said vapor while stillat a low subatmospheric temperature; liquefying a portion of said vaporand thereby providing reflux liquid for said fractionation; passing theremainder of said vapor while at substantially the temperature ofwithdrawal in heat interchange with a stream of the natural gas to berefrigerated and venting the resultant warmed vapor, withdrawing saidhydrocarbonenriched liquid and vaporizing it to produce a gas having asmaller nitrogen content than the original natural gas.

6. The method of claim 4 further characterized in that the liquefactionof divided vapor is carried out in part by heat exchange with a boilingliquid refrigerant.

7. The method of claim 4 wherein the liquefaction of said dividedportion of vapor is carried out in part by heat exchange with a boilingliquid comprising predominantly methane.

8. A manipulation of natural hydrocarbon gas initially contaminated withnitrogen, comprising: refrigerating said gas; fractionating therefrigerated material by repeated contacts between downowing liquid andupowing vapor and thereby separating a vapor enriched in nitrogen from aliquid enriched in hydrocarbons, dividing last said vapor into twoportions, compressing a first portion to form a Warmer compressed vapor,passing said first portion of vapor in indirect heat exchange with saidwarmer compressed vapor thereby heating said first portion and coolingsaid compressed vapor, further cooling said cornpressed vapor to liquefyit and supplying a portion of said liquefied vapor as reux for saidfractionation, passing the second portion of divided vapor while atsubstantially the temperature of withdrawal in heat exchange with astream of the natural gas to be refrigerated and venting the resultantwarmed vapor; withdrawing said hydrocarbon-enriched liquid from saidfractionation and vaporizing it to produce a gas of lower nitrogencontent than the original natural gas.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 668,197 Le Sueur Feb. 19, 1901 1,843,043 Patart lan. 26, 19321,922,573 Dunkak Aug. 15, 1933 1,931,791 Dueringer Oct. 24, 19332,080,163 Twomey Aug. 17, 1937 2,180,090 Mesinger Nov. 14, 19392,258,015 Keith et al. Oct. 7, 1941 2,265,527 Hill Dec. 9, 19412,464,891 Rice May 22, 1949 2,471,602 Arnold May 3l, 1949 2,475,957Gilmore July 12, 1949 2,500,118 Cooper Mar. 7, 1950 2,525,802 JoerrenOct. 17, 1950 2,535,148 Martin et al Dec. 26, 1950 2,541,569 Born et al.Feb. 13, 1951 2,557,171 Bodle et al June 19, 1951

